Fiber optic particle motion sensor system

ABSTRACT

A sensor apparatus combines an optical sensor in which acceleration, acoustic velocity, or displacement (vibration) causes a corresponding shift in the center wavelength of the sensor output, coupled to a high speed interferometric interrogator, through an unbalanced fiber interferometer. The unbalanced interferometer functions to translate optical wavelength shift into phase shift, which is easily demodulated by the interrogator.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of provisional application No.60/999,246 filed

Oct. 16, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to methods and systems for acquiringacceleration and/or velocity data using fiber optic sensors.Specifically, the invention relates to grating sensors with ultra narrowband gratings, combined with interferometric wavelength-to-phaseconversion and low noise interferometric interrogation.

2. Background of the Invention

There are many applications that require a device to measure the dynamicacceleration or acoustic velocity signal at a given location. Examplesinclude: the seismic exploration/monitoring of oilfields, seismicmonitoring for earthquakes, structural integrity monitoring, and healthmonitoring of vibrating equipment/machinery acoustic monitoring inmarine environments (e.g., SONAR). For decades, such monitoring has beenalmost exclusively performed using electronic-based sensors such aspiezoelectric sensors and magnet/coil sensors. These sensors typicallygenerate a voltage output that is proportional to the intensity of theapplied vibratory motion (displacement, velocity or acceleration).Because the generated voltage levels are relatively weak (i.e., lowlevel), electronics are required for amplification, signal conditioning,filtering, and in most cases digitization/multiplexing. Theseelectronics must be located very close to the sensor to limit theintroduction of noise into the system. Thus, the electronics must bedesigned to operate in the local environment(temperature/vibration/humidity/shock) where the sensor is placed.

Recently, the use of fiber optic sensors has become more prevalent forsensing applications, particularly in those applications where thesensors must be placed in harsh environments, which seriously affectsthe performance/reliability of the associated electronics. Fiber opticsensors have an advantage in that they require no electronics at or nearthe sensor. In fiber optic sensors, light is sent through the opticalfiber from a remote location (in a benign environment). The measurandcauses a change in the optical transmissive property of the fiber whichis then detected as a change in the received light signal at the remoteelectronics.

Fiber optic sensors generally fall into two categories, those designedfor making high speed dynamic measurements, and those designed for lowspeed, relatively static measurements. Examples of dynamic sensorsinclude hydrophones, geophones, and acoustic velocity sensors, where thesignal varies at a rate of 1 Hz and above. Examples of low speed(static) sensors include temperature, hydrostatic pressure, andstructural strain, where the rate of signal change may be on the orderof minutes or hours. This invention relates primarily to dynamicmeasurements of acceleration, acoustic velocity, and vibration usingfiber optic sensors. Historically, such sensors have been more costlythan the legacy electronic versions because they are difficult tomanufacture, require complicated and expensive equipment for evenlimited automated assembly, and involve significant amounts of skilledtouch labor to produce. Although fiber Bragg grating (FBG)accelerometers are currently available, they incorporate spectroscopicinterrogation, which limits the sensitivity to about 1 mg. However, manyapplications require sensitivities on the order of 30-50 ng. Fiber laserdevices have also been used for sensing. However, they are expensive andtend to be unstable. The invention endeavors to solve these problems andmore to provide extremely high sensitivity acceleration measurementssuitable for a wide range of applications requiring sensors inenvironments in which electronics often cannot survive.

SUMMARY OF INVENTION

The present invention provides a fiber optic sensing system with a levelof performance several orders of magnitude higher than is otherwiseachievable using prior art technologies. The system combines an FBGsensor packaged as a ‘particle motion sensor,’ such that acceleration,acoustic velocity, or displacement (vibration) cause a correspondingshift in the center wavelength of the FBG reflection (or transmission)spectrum, coupled to a high speed interferometric interrogator, throughan unbalanced fiber interferometer. The unbalanced interferometerfunctions to translate the FBG wavelength shift into a phase shift,which is easily demodulated by the interrogator, i.e., the wavelengthshift of an FBG sensor is detected by utilizing the inherent wavelengthdependence of an unbalanced fiber interferometer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a particle motion sensing system inaccordance with an embodiment of the invention.

FIG. 2A is a cross-sectional view of a sensor suitable for use in thesystem of FIG. 1.

FIGS. 2B and 2C show details of the circular hinge.

FIGS. 3A and 3B show two embodiments of the narrow linewidth grating.

FIG. 4 is a transmission spectrum of a phase shifted grating.

FIG. 5 is a block diagram of the source optics.

FIG. 6 is a schematic of the receive optics.

FIG. 7 is a diagram of an embodiment of the ASE filter.

FIG. 8 is a block diagram of a closed loop interferometric interrogator.

FIG. 9 is a block diagram of a WDM/TDM multiplexed system.

FIG. 10 is a block diagram of the source optics of a WDM/TDM multiplexedsystem.

FIG. 11 is a block diagram of the receive optics of a WDM/TDMmultiplexed system.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods anddevices are omitted so as to not obscure the description of the presentinvention with unnecessary detail.

A particle motion sensing system 10 according to one embodiment of thepresent invention is shown in FIG. 1. The particle motion sensing system10 includes a transducer or sensor 100, source optics 200, receiveoptics 300, an interferometric interrogator 400 and signalprocessing/recording electronics 500.

Although a number of different configurations of the sensor 100 may beemployed, FIG. 2A shows an exemplary embodiment for use with narrow bandgratings. Sensor 100 includes a housing 110, an optical fiber 130, aproof mass 150, and a pretension spring 170. The optical fiber 130 has afree region 132 in which grating 135 is inscribed. The optical fiber 135is attached at one end to the housing 110 by means of a first anchor 120and at the other end to the proof mass 150 by means of a second anchor160. The optical fiber 130 may be attached to the first anchor 120 andthe second anchor 160 by bonding or any other suitable method forpreventing the optical fiber 130 from slipping relative to either thefirst anchor 120 or the second anchor 160. Both the first anchor 120and/or the second anchor 160 may be round spool-shaped structuresforming a capstan to help secure the optical fiber 130 to it with thefriction therebetween caused by wrapping the optical fiber 130 aroundthe outer diameter of the first anchor 120 or second anchor 160. Theproof mass 150 is suspended from the housing 110 by means of suspensionmember 180, a clamping ring 140, standoffs 145, screws 147 and nuts 148.

Motion of the sensor 100 is identical to motion of the housing 110.Motion of the sensor along direction 112 results in motion of thehousing 110 relative to the proof mass 150. Relative motion between thehousing 110 and the proof mass 150 is constrained to occur only in thedirection 112 by the suspension member 180. Relative motion between thehousing 110 and the proof mass 150 along direction 112 is controlled bythe fiber 130 and the pretension spring 170. Pretension spring 170controls the quiescent tension on the fiber 130 in conjunction with themass of the proof mass 150. The force applied between the housing 110and the proof mass 150 by the pretension spring 170 is controlled by aflexible cantilever 175 and an adjustment screw 177. The flexiblecantilever 175 is permanently attached at one end to the housing 110.

Referring to FIGS. 2B and 2C, the suspension member 180 comprises one ormore flexible circular membranes or diaphragms fabricated by stamping orforming a flat stock of ductile metal to form a series of concentricwaves 185. These waves 185 allow the central region 182 of suspensionmember 180 to move with little resistance along direction 112 relativeto outer portion 183 of suspension member 180 while ensuring centralportion 182 and outer portion 183 of suspension member 180 remainparallel when the proof mass 150 is sandwiched between a pair ofsuspension member 180. Thus, for small amplitude motions, motion of theproof mass is allowed along direction 112, but resisted in all otherdirections, including rotational motions.

Referring to FIG. 3A, the grating 135 is created by fabricating two FBGs1050, each of which is a periodic change of the refractive index of theglass core 133 of the optical fiber 130 by means of a laser, a phasemask, an interferometer or other methods well known to practitioners inthe art. The two FBGs are separated by a small space 1060 on the orderof 100 microns. Alternatively, as shown in FIG. 3B, the grating 135′ canbe fabricated as a single grating comprising two halves 1065 and 1070which are shifted in phase relative to one another, for example by itradians. The resulting phase-shifted grating has a typical transmissionspectrum 1005 shown in FIG. 4. The significant features of thetransmission spectrum 1005 are a central peak 1000, two stop bands 1010and two pass bands 1020. Typical values for the spectrum 1005 are a peaktransmission width of 0.4 pm, a stop band 1010 depth of >40 dB, stopband 1010 width of about 800 pm and near 100% transmission in the passbands 1020.

Relative motion between the housing 110 and the proof mass 150 changesthe longitudinal strain within the free region 132 of optical fiber 130between the first anchor 120 and the second anchor 160. Changes in thelongitudinal strain within the fiber 130 cause a proportional shift ofthe peak wavelength of the reflection or transmission spectrum of thegrating 135.

Referring to FIG. 5, the source optics 200 includes a broadband opticalsource 210, prefilters 220 and an optical amplifier 230. In theexemplary embodiment of the invention, the broadband optical source 210is a Superluminescent Light Emitting Diode (SLED). However, any suitableoptical source with a bandwidth of at least approximately 1 nm may beused, such as an Amplified Spontaneous Emission (ASE) source, LightEmitting Diode, (LED), etc. The source should provide an intensity of atleast 0.4 mW/nm into an optical fiber and have a spectral output atleast 1 nm wide. The output of the broadband optical source 210 isconnected to the input of the prefilters 220 through optical fiber 215.The prefilters 220 may comprise one or more band pass optical filters,each of which has a passband of about 1 nm. Examples of such a filterare a Dense Wavelength Division Multiplexer (DWDM) or an Optical AddDrop Multiplexer (OADM), both of which are well known to those practicedin the art of telecommunication and sensing optics. The output of theprefilters 220 is connected to the input of the optical amplifier 230through optical fiber 225. The optical amplifier 230 can be any suitablemeans for providing optical gain. Examples of appropriate opticalamplifiers are Erbium-Doped Fiber Amplifiers (EDFAs) and SemiconductorOptical Amplifiers (SOAs), both of which are well known to thosepracticed in the art of telecommunication and sensing optics. The outputof the optical amplifier 230 is connected to the input of the sensor 100through optical fiber 235.

Referring to FIG. 6, the receive optics include an Amplified SpontaneousEmission (ASE) filter 305 and a mismatched path interferometer 310. Theoutput of the sensor 100 is connected to the input of the ASE filter 305through optical fiber 302. The ASE filter 305 is a bandpass filter usedto minimize the intensity of amplified spontaneous emission from theoptical amplifier 230 that is outside the stop band 1010 of the grating135. The ASE filter 305 preferably has a very narrow transmissionpassband. An example of an appropriate ASE filter 305 is a 50 GHz OADM.

Details of ASE filter 305 are shown in FIG. 7. ASE filter 305 includesan optical circulator 303 and an FBG. The optical circulator 303 is apassive optical device well known within the field of telecommunicationsthat passes light from a first port 309 to second port 308, but not viceversa. It also passes light from second port 308 to third port 311, butnot vice versa. It also does not pass light from third port 311 to firstport 309. In other words, light can only circulate in and out of thecirculator 303 in one direction. Connected to output power of thecirculator 303 is the FBG 304. The FBG 304 has a high peak reflectivity(>80%) and a full width half maximum bandwidth of about 300 pm. Suchdevices are well known to those who practice in the art. The distal leadof FBG 304 remains unconnected.

Referring again to FIG. 6, the mismatched path interferometer 310includes a 2×2 optical coupler 320, a phase modulator 330, an opticaldelay line 340 and two mirrors 350. The input leg 307 of the 2×2 opticalcoupler 321 is connected to the output of the ASE filter 305. The 2×2optical coupler 321 divides the input light with half going to each ofits output leads 325 and 337. One output lead 325 is connected to aphase modulator 330, which is connected to mirror 350 through opticalfiber 335. The phase modulator 330 is used to impose a known phase tothe light traveling within leg 370 of the mismatched path interferometer310. The other output lead 337 of the 2×2 optical coupler 320 isconnected to an optical delay line 340, which is connected to mirror 350through optical fiber 345. The physical length difference between thetwo legs 370 and 380 of the mismatched path interferometer 310 isnon-zero, and is preferably in the range of approximately 1-5 meters.

The mismatched pathlength interferometer 310 converts the changing peakwavelength in the central peak 1000 of the light transmitted from thesensor 100 into a change in phase angle of the light traversing the twolegs 370 and 380. The conversion of the peak wavelength to phase is onthe order of 2 rad/pm, and increases with larger differences in lengthbetween the two legs 370 and 380.

After the light passes through the mismatched pathlength interferometer310, it travels by means of output fiber 355 to the interferometricinterrogator 400. The function of the interferometric interrogator is tomeasure the change in the phase angle difference between the two legs370 and 380 of the mismatched pathlength interferometer 310 over time. Anumber of approaches have been used for interferometric interrogation,such as heterodyne demodulation and homodyne demodulation. For example,the Optiphase OPD-4000 is a suitable demodulator. It applies asinusoidal modulation waveform to the phase modulator 330. An examplefrequency for the modulation waveform is 20 kHz, well above the plannedmaximum operational frequency of the system—about 150 Hz. The resultantmodulated optical waveform that arrives at the interferometricdemodulator 400 is converted to an electrical signal, digitized anddownconverted within the interferometric demodulator 400.

FIG. 8 illustrates a low noise method of measuring the phase angledifference between the two legs 370 and 380 of the mismatched pathlengthinterferometer over time using a closed loop interferometricinterrogator. A stable, low noise local oscillator 460 provides amodulation waveform such as a sine wave. A bias amplifier 470 adjuststhe amplitude of the output of the local oscillator 460 to be applied tothe phase modulator. Ideally, a π/2 radian phase shift is applied to thephase modulator 330 to ensure that the mismatched pathlengthinterferometer 310 operates within a roughly linear range of itstransfer function.

The interference signal from the mismatched pathlength interferometer310 travels along optical fiber 411 and illuminates photodetector 410.The purpose of photodetector 410 is to convert light into an electricalcurrent. A number of suitable devices are available for photodetector410. The exemplary embodiment utilizes an ETX-100, manufactured by JDSUniphase. The electrical output of the photodetector is connected to avery low noise, high gain preamplifier 420. The output of thepreamplifier is connected to Automatic Gain Control (AGC) 430. The AGC430 enables continuous correction for changes in optical intensitylevels throughout the system. The output of AGC 430 is mixed with thesignal from the local oscillator 460 within the analog multiplier 440.The purpose of the analog multiplier 440 is to provide a pair of signalsequal to the sum and difference of the AGC 430 output and localoscillator 460. The output of the analog multiplier 440 is connected tothe input of a low pass filter 450. For a 150 Hz maximum frequency rangesystem, the cutoff frequency of the low pass filter 450 would be around500 Hz. The cutoff frequency of the low pass filter is well below thesum frequency of the output of the analog multiplier 440. This ensuresonly the low frequency difference signal from the analog multiplier 440is passed. The combination of local oscillator 460, analog multiplier440 and low pass filter 450 function as a synchronous detector. Theoutput signal from the low pass filter 450 is passed along to high gainamplifier 455. The output of the high gain amplifier 455 is connected tothe input of the variable gain output driver amplifier 495 whichprovides a voltage output proportional to the phase angle differencebetween the two legs 370 and 380 of the mismatched pathlengthinterferometer 310 over time. The output voltage of the amplifier 495 isalso proportional to the amplitude of the acceleration experienced bythe sensor 100.

The output of the bias amplifier 470 is added to the output of the highgain amplifier 455 in a summing amplifier 480. The output of the summingamplifier is connected to the input of a modulator driver amplifier 490.The output 491 of the modulator driver amplifier 490 is applied toelectrical input 331 of the phase modulator 330 within the mismatchedpathlength interferometer 310 (FIG. 6).

The negative overall loop gain of the interferometric interrogator 400acts to provide negative feedback to the phase modulator which is equaland opposite to the optical phase angle difference between the two legs370 and 380 of the mismatched pathlength interferometer 310. Thisnulling action serves to maintain operation of the mismatched pathlengthinterferometer 310 within the linear range of its transfer function.

The operation of the particle motion sensing system 10 is thereforegoverned by the following scale factor equation:

SF _(system) =SF _(sensor) *SF _(FBG) *SF _(interferometer)

Where the overall system scale factor SF_(system) is the product of thesensor scale factor SF_(sensor), typically 1000 microstrain/g, the FBGscale factor SF_(FBG), typically 1.2 pm/microstrain, and theinterferometer scale factor SF_(interferometer), typically about 3Rad/pm. These typical values result in an overall system scale factor of2,988 rad/g (69.5 dB:Rad/g). The dominant noise source in these types ofsystems is the Relative Intensity Noise (RIN) caused by the extremefiltering of the broadband optical source 210 by the FBG 135. Thisresults in a phase noise floor of about −80 dB:rad/√Hz. Therefore, theresulting noise floor would be −80 dB-69.5 dB=−149.5 dB:g/√Hz. Fornormalized detection within a 1 Hz bandwidth, this provides a minimumdetectable acceleration of −149.5 dB:g or about 33 ng, which is typicalperformance for electronic, moving coil-type geophones, but about 10,000times better resolution than FBG accelerometers that employ typical, orspectroscopic-type interrogation.

Practical systems frequently require a number of sensors be combined andprocessed with a single set of electronics. Mutiplexing multiple sensorsis easily accomplished with interferometric FBG acceleration sensing.One such embodiment is a hybrid Wavelength Division Multiplexing(WDM)/Time Division Multiplexing (TDM) multiplexed system such as thatshown in FIG. 9, which is simplified for a 4 sensor system. It will berecognized that the same principles apply to larger arrays of sensors.

An embodiment of a WDM/TDM multiplexed system 2000 is shown in FIG. 9.This system includes source optics 2100, which is shown in greaterdetail in FIG. 10. The output of a broadband optical source 2110 isconnected to the input of an optical switch 2113. Semiconductor OpticalAmplifiers (SOAs) are typical devices suitable for high extinction ratiooptical switching. Suitable devices are manufactured by companies suchas Inphenix and Kamelian. The optical switch 2113 creates a series ofpulses needed for interrogation. Dense Wavelength Division Multiplexer(DWDM) 2115 divides the light along multiple fibers 2120, each with adifferent central wavelength, typically separated by about 0.8 nm. Alongeach of the fibers 2120 is added a different fiber optic delay line 2116through 2119, typically 50 to 100 m. The four different wavelengths oflight travelling through the delay lines 2116 through 2119 are passedthrough a second DWDM 2135, which recombines all four wavelengths andoutputs them together along optical fiber 2125 to an optical amplifier2130. The output of the optical amplifier 2130 passes through opticalfiber 2170.

Referring back to FIG. 9, the output of the source optics 2100 passesthrough optical fiber 2170 to the sensor array 2150. The sensor array2150 consists of a series of sensors and filters in a ladderconfiguration with one downlink optical fiber and one uplink opticalfiber. Light travelling from optical fiber 2170 continues along downlinkoptical fiber 2175 to OADM 2200. OADM 2200 acts to filter out a narrow(on the order of 1 nm wide) wavelength band of light for the firstsensor and passes the remainder of the light for the remaining sensors.The ‘drop’ leg of OADM 2200 is connected to the input of sensor 2210.The output of sensor 2210 is connected to the ‘add’ leg of OADM 2250.The ‘pass’ leg of OADM 2250 is connected to the uplink fiber 2255. Thelight from the sensor 2210 thus passes along the uplink optical fiber2255 to the receive optics 2260.

The light from the ‘pass’ leg of OADM 2200 is connected to the input ofOADM 2220. OADMs 2200, 2220, 2330 and 2420 have different addwavelengths. OADMs 2200, 2220, 2330 and 2420 have different passwavelengths. The ‘drop’ leg of OADM 2220 is connected to sensor 2230.The output of sensor 2230 is connected to the ‘add’ leg of OADM 2240.The ‘pass’ leg of OADM is connected to the input leg of OADM 2250. The‘pass’ leg of OADM 2220 is connected to the input leg of OADM 2320. The‘drop’ leg of OADM 2320 is connected to the input of sensor 2325. Theoutput of sensor 2325 is connected to the ‘add’ leg of OADM 2350. The‘pass’ leg of OADM is connected to the input leg of OADM 2240. The‘pass’ leg of OADM 2320 is connected to the input leg of OADM 2420. The‘drop leg of OADM 2420 is connected to the input of sensor 2425. Theoutput of sensor 2425 is connected to the ‘add’ leg of OADM 2450. The‘pass’ leg of OADM 2450 is connected to the input leg of OADM 2350. The‘pass’ leg of OADM 2420 and the input leg of OADM 2450 remainunconnected.

Referring to FIG. 11, the uplink optical fiber 2255 is connected to theinput of DWDM 2400. DWDM 2400 divides the light into four bands, one foreach of the sensors 2210, 2230, 2325 and 2425. Each output leg of theDWDM 2400 is connected to a respective one of four ASE filters 2410,2420, 2430 and 2440. The ASE filters are identical to ASE filter 305.The outputs of the ASE filters 2410, 2420, 2430 and 2440 are connectedto the four inputs of DWDM 2460, which recombines the wavelengths onto asingle fiber 2465. Fiber 2465 is connected to the mismatched pathlengthinterferometer 2470. The output of the mismatched pathlengthinterferometer 2470 is connected to fiber 2265.

Referring again to FIG. 9, fiber 2265 is connected to TDM demodulator2300. A number of different TDM demodulators are available, such as theERS-5100 manufactured by Optiphase, Inc., Van Nuys, Calif. The TDMdemodulator 2300 controls the optical switch 213, which provides lightpulses to each of the sensors 2210, 2230, 2325 and 2425 that areseparated in time such that each sensor can be interrogated separatelyby the same TDM demodulator 2300. The TDM demodulator 2300 also controlsthe amplitude and phase of the phase modulator within the mismatchedpathlength interferometer 2470, which is identical to the mismatchedpathlength interferometer 380 used for a single sensor 100. The outputof the TDM demodulator 2300 is a digital representation of the output ofeach of the sensors 2210, 2230, 2325 and 2425 and is input to the signalprocessing/recording electronics 2500 for further filtering, averaging,storage and display.

In general, it will be recognized that the above-described invention maybe embodied in other specific forms without departing from the spirit oressential characteristics of the disclosure. Thus, it is understood thatthe invention is not to be limited by the foregoing illustrativedetails, but rather is to be defined by the appended claims.

1. A sensor apparatus comprising: a sensor containing a section of an optical fiber having at least one grating inscribed therein, the grating characterized by a reflection or transmission peak having a width≦100 picometers, wherein a central wavelength of the grating reflection or transmission peak changes in response to an input to the sensor; an optical light source coupled to the optical fiber, the light source having a bandwidth ≧1 nanometer; an interferometer coupled to the optical fiber with a pathlength mismatch equal to or less than a coherence length of light leaving the grating, wherein the interferometer converts changes in wavelength of light from the sensor into changes in optical phase; an interrogator coupled to an output of the interferometer providing an electrical output proportional to the input to the sensor.
 2. The sensor apparatus of claim 1 wherein the grating comprises a fiber Bragg grating.
 3. The sensor apparatus of claim 1 wherein the grating comprises a phase shifted grating.
 4. The sensor apparatus of claim 1 wherein the grating comprises a pair of gratings comprising a Fabry-Perot interferometer.
 5. The sensor apparatus of claim 1 wherein the light source comprises a Superluminescent Light Emitting Diode.
 6. The sensor apparatus of claim 5 wherein the light source further comprises a bandpass filter.
 7. The sensor apparatus of claim 6 wherein the light source further comprises an optical amplifier.
 8. The sensor apparatus of claim 1 wherein the interferometer is a Michelson interferometer.
 9. The sensor apparatus of claim 1 wherein the interrogator is a closed loop interrogator wherein the phase change in the interferometer creates an error signal that is nulled and wherein the null signal is an output proportional to the phase change.
 10. The sensor apparatus of claim 9 wherein the closed loop interrogator comprises a photodiode, an amplifier, a local oscillator, a synchronous detector, connection to a phase modulator and an output.
 11. The sensor apparatus of claim 1 further comprising a filter disposed between the output end of the optical fiber and the interferometer.
 12. The sensor apparatus of claim 11 wherein the filter is a bandpass filter.
 13. The sensor apparatus of claim 11 wherein the filter is an Optical Add Drop Multiplexer.
 14. The sensor apparatus of claim 11 wherein the filter is a Dense Wavelength Division Multiplexer.
 15. The sensor apparatus of claim 11 wherein the filter is an amplified spontaneous emission filter.
 16. The sensor apparatus of claim 1 wherein the sensor is one of a plurality of sensors and further comprising source optics for producing light signals at a plurality of discrete wavelengths and receive optics for processing combined signals from each of the sensors.
 17. The sensor apparatus of claim 16 wherein the source optics comprises a first Dense Wavelength Division Multiplexer having a plurality of outputs, each coupled to a respective one of a plurality of fiber optic delay lines, and a second Dense Wavelength Division Multiplexer for combining outputs of the plurality of delay lines.
 18. The sensor apparatus of claim 16 wherein the receive optics comprises a third Dense Wavelength Division Multiplexer having a plurality of outputs, each coupled to a respective one of a plurality of filters, and a fourth Dense Wavelength Division Multiplexer for combining outputs of the plurality of filters.
 19. A sensor comprising: a housing; a mass moveably disposed within the housing; a first anchor attached to the housing; a second anchor attached to the mass; at least one suspension member interposed between the mass and the housing that allows the mass to move freely along one axis while restricting motions of the mass along all other axes; a section of an optical fiber attached at one end thereof to the first anchor and attached at the other end thereof to the second anchor, the section of optical fiber having a grating inscribed therein, the section of optical fiber serving both as a restoring spring and to sense relative motion between the mass and the housing; an additional spring between the mass and the housing for imposing a fixed tension to the fiber to enable measurements of motion of the sensor in any orientation; means for adjusting the spring to control the fixed tension imposed upon the fiber.
 20. The sensor of claim 19 wherein the suspension member comprises at least one circular diaphragm.
 21. The sensor of claim 20 wherein the circular diaphragm contains at least one concentric ridge.
 22. The sensor of claim 19 wherein the first anchor comprises a capstan.
 23. The sensor of claim 19 wherein the second anchor comprises a capstan.
 24. The sensor of claim 19 wherein the grating comprises a Bragg grating.
 25. The sensor of claim 19 wherein the grating comprises a phase shifted grating.
 26. The sensor of claim 19 wherein the grating comprises a pair of gratings comprising a Fabry-Perot interferometer.
 27. The sensor of claim 19 wherein the grating has a central transmission peak.
 28. The sensor of claim 19 wherein the grating has a transmission peak with a spectral width of approximately 0.3 picometer.
 29. The sensor of claim 19 wherein the grating has a stop band of low transmission extending above and below a wavelength of the peak transmission.
 30. The sensor of claim 19 wherein the grating has a stop bandwidth of approximately 1 nanometer.
 31. The sensor of claim 19 wherein the mass is coupled to the housing by at least one flexible joint that allows free movement in one orthogonal axis and limits movement in the other two axes.
 32. The sensor of claim 31 wherein said at least one flexible joint comprises a circular diaphragm.
 33. A method of measuring acceleration comprising: providing a sensor containing a section of an optical fiber having a grating inscribed therein, illuminating a first end of the optical fiber with a light source; coupling a second end of the optical fiber to an interferometer; subjecting the sensor to an acceleration; converting the acceleration to longitudinal strain within the fiber grating; converting the longitudinal strain to a peak in the grating reflection or transmission spectrum within the fiber grating; converting the change in the peak in the grating reflection or transmission into a phase shift within the interferometer; measuring an intensity output from interferometer, in which the intensity is proportional to the phase shift in the interferometer and therefore, proportional to the sensed acceleration.
 34. The method of claim 33 wherein the grating comprises a Bragg grating.
 35. The method of claim 33 wherein the grating comprises a phase shifted grating.
 36. The method of claim 33 wherein the grating comprises a pair of gratings comprising a Fabry-Perot interferometer.
 37. The method of claim 33 wherein the grating comprises a pair of gratings that are phase shifted relative to one another.
 38. The method of claim 37 wherein the phase shift between the gratings is π radians.
 39. The method of claim 33 wherein the grating has a central transmission peak.
 40. The method of claim 33 wherein the grating has a transmission peak with a spectral width of approximately 0.3 picometer.
 41. The method of claim 33 wherein the grating has a stop band of low transmission extending above and below a wavelength of the peak transmission.
 42. The method of claim 33 wherein the grating has a stop bandwidth of approximately 1 nanometer.
 43. The method of claim 30 wherein the phase shift is measured using an interferometric interrogator coupled to the interferometer.
 44. The method of claim 43 wherein the interferometric interrogator is a closed loop interrogator.
 45. The method of claim 43 wherein the interferometric interrogator is a demodulator. 